It's fine, the original question states: Create your own term using a rational exponent and then convert it to a radical expression. Explain, in complete sentences, how the expression was converted.
You created the term cbrt(27), they want yo to be able to convert to rational exponents, so 27^(1/3)
Then just explain how you converted it.

I gottta go to bed... :( i think you can handle the rest??
It's not a formula, its just the degree of the radical goes to the denominator of the rational exponent, and the degree of the radicand goes to the numerator of the rational exponent.
http://www.purplemath.com/modules/exponent5.htm